Polyurethanes (PUs) have been extensively studied for use in medical applications such as pacemaker leads, blood bags, catheters, bladders, artificial hearts, vascular grafts and synthetic heart valves.
Although there are a number of formulations offering a great variety of chemical and physical properties, they are mostly thermoplastic compositions and there remain two major drawbacks of PUs used in medical applications: they are susceptible to certain conditions that can cause chemical degradation of properties in vivo and they may be prone to excessive strain relaxation (creep) especially where cyclic stress is incurred, such as in cardiovascular prostheses.
There have been a number of attempts to improve the chemical and mechanical properties of PUs. As the chemical stability of PUs is closely linked to the chemical nature of the macroglycol units constituting soft segments of PUs, the polyester macroglycols initially used and found to be hydrolytically unstable in vivo, were replaced with soft segments based on polyethers. However, subsequent studies showed that although the polyether segments were hydrolytically stable, certain polyether urethanes were susceptible to oxidative degradation in the presence of metal ions and other strong oxidants present in physiological media. Further studies toward full or partial replacement of the polyether functionalities with carbonates, siloxanes, hydrocarbons and fluorinated ethers continue.
The use of polycarbonate soft segments has been reported to increase chemical stability, but have been shown to still be susceptible to hydrolytic and oxidative degradation. Although siloxane based polyurethane materials were initially thought to be both hydrolytically and oxidatively stable, they have also recently been shown to degrade in aqueous media. Siloxane soft segment-based PUs also tend to have lower toughness than their polyether soft-segment counterparts. Medtronic Inc. (U.S. Pat. No. 4,873,308) eliminated the use of polyester, polyether and polycarbonates by basing an experimental urethane on hydrocarbon (dimerol) soft segments. Dimerols are derived from dimer acids: The dimer acids as the term is used herein are described and discussed in the book “The Dimer Acids”, edited by Edward C. Leonard, published by Humko Sheffield Chemical, 1975. Dimer acids are the reaction product of a Diels-Alder addition or other coupling process of two aliphatic, unsaturated, predominantly 18 carbon fatty acids. Dimer acids take the form of single ring, double ring or branched chain structures predominantly having two carboxylic acid functionalities. This modification indeed produced enhanced biostability, but being uncrosslinked polymers, they would still be prone to significant plastic deformation.
In this specification, “living hinge” is defined as a polymer which induces a yield strain in the hinge zone. A yield point can be defined as an increase in strain without a corresponding increase in stress. It is an initial extension of the polymer specimen beyond its elastic limit and this results in an alignment of the polymer molecules in the hinge which gives enhanced flex life. It results in a necking down relative to the thickness of the specimen on either side. The patent by Moe (U.S. Pat. No. 6,117,169) which generally discloses a heart valve having a living hinge attachment describes heart valves where the valve cusps are integral with the valve sheath and thus appears to be different from this definition. Silicone polymers sometimes described in synthetic valve designs, (Reference V. M. Parfeev, I. V. Grushetskii, E. V. Smurova: “Mechanical Properties of Elastomers for Artificial Leaflet Heart Valves,” Mechanics of Composite Materials, January-February, 1983, Volume 19, Issue 1, pp 92-99) generally do not have a true yield point and it is also uncommon for polyurethanes to have one.
There is thus a need for chemically and mechanically stable linear and crosslinked polyurethane polymers for use as medical implant devices that alleviate some of the above mentioned problems.
The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.